(1). Field of the Invention
The present invention relates to a mechanical assembly provided with means for monitoring for a structural anomaly, to a gearbox provided with such a mechanical assembly, and to a method of monitoring for a structural anomaly, which method is implemented by said mechanical assembly.
More particularly, the invention relates to the field of gearboxes for aircraft, and in particular for rotorcraft of the helicopter type.
(2). Description of Related Art
A rotorcraft includes a power plant for setting its rotary wing into rotation. The power plant is provided with one or more engines for driving the gears of a gearbox, in particular a helicopter main gearbox that drives the rotary wing of the helicopter in rotation.
The main gearbox comprises a mechanical assembly having moving mechanical elements in contact with one another, e.g. elements of the gear type. Conventionally, the mechanical assembly includes a first shaft set into rotation by the power plant, the first shaft being secured to a sun gear of epicyclic speed-reduction means.
The sun gear then co-operates with a plurality of planet gears of said speed-reduction means. Each planet gear comprises a toothed body surrounding ball or roller rolling connection means fastened to a pin, the pin of the planet gear being secured to a planet carrier, the planet carrier being secured to a second shaft possibly constituting the mast of a rotorcraft rotor.
It should be observed that in the present specification, the term “rolling connection means” is used to designate a ball bearing or a roller bearing or any other known type of rolling bearing.
Under such circumstances, each planet gear moves in rotation both about its own first axis of rotation and also about a second axis of rotation of the planet carrier.
Furthermore, the mechanical assembly includes a toothed outer annulus with an inner periphery of the outer annulus co-operating with each of the planet gears. Each planet gear moves in an annular space situated between the inner periphery of the outer annulus and the sun gear.
It is found that the mechanical assembly is liable to deteriorate over time, for example cracks may appear in the body or the rolling connection means of a planet gear.
Visual inspections may be organized to detect any cracks, with damaged parts then being replaced. Similarly, it is also possible to search for the presence of metal particles in the liquid used for lubricating the mechanical assembly.
In order to facilitate crack detection, a technique is known that implements at least one accelerometer arranged on an outer periphery of the annulus that does not face the annular space. Each accelerometer is connected to a control member that receives and analyzes the signal coming from the accelerometer. It can be understood that the accelerometer is spaced apart from each of the components of a planet gear, and in particular from its rolling connection means by a distance that varies while said moving members are rotating.
The passage of a mechanical element over a crack gives rise to vibration that can be detected by an accelerometer. For example, the passage of a ball or a roller forming part of rolling connection means against a crack that exists in the body of a planet gear generates vibration. Thus, if the control member receives a crack-representing signal coming from an accelerometer, it is in a position to issue an alarm.
That device is very effective. Nevertheless, the vibration emitted by a ball or a roller passing over a crack is of very small amplitude. Because of the varying distance between the sensor of an accelerometer and the ball or roller at the time of said passage, there is a risk that the accelerometer will be too far away and that the control member will not detect said vibration.
Document EP 0 889 314 presents a method of monitoring a transmission assembly implemented by such a monitoring device, the method consisting in:                acquiring a signal from an accelerometer;        calculating a frequency domain transform of said signal in order to obtain a sequence of samples;        acquiring a sample in said sequence of samples, calculating a real amplitude value of said acquired sample;        calculating a relative difference between said real amplitude value and a reference amplitude value; and        comparing said relative difference with at least a first predetermined threshold.        
Similarly, document EP 0 899 553 describes a method of monitoring a transmission assembly implemented by such a monitoring device, and consisting in:                acquiring a signal from an acceleration sensor;        sampling said signal at a sampling frequency in order to obtain an initial sample sequence;        organizing the samples present in said initial sequence into groups, each group being defined by a predetermined number of samples, said sample groups being ordered in a succession and each of them having a respective position in said succession;        performing first filtering of said succession of sample groups in order to obtain an intermediate sample sequence; and        processing said first intermediate sample sequence in order to obtain a final sample sequence.        
The technological background includes in particular the following patent documents: WO 2007/085259, US 2004/200283, and DE 4 239 096.
For example, document DE 4 239 096 describes a device for monitoring twisting forces. That device is provided with a strain gauge transmitting signals to a receiver member via a wireless connection.